1. Field of the Invention
This invention relates to an edge detection method.
2. Description of the Related Art
When the edge of a substrate is detected, if the substrate is opaque, the edge can be detected easily by an optical method. Specifically, when a region including the edge of the substrate is irradiated with measuring light, the measuring light is blocked by the substrate in the region where the substrate exists, with the result that the light does not reach a light detecting part (or a light-receiving part). The edge can be detected by detecting a region where the measuring light does not reach the light detecting part (or a dark region). However, in the case of a transparent substrate, since measuring light passes through the transparent substrate, the dark region cannot be detected. Accordingly, it is not easy to detect the edge of a transparent substrate by an optical method.
To overcome the above problem, a method of beveling the peripheral part of the transparent substrate to form an inclined surface and refracting measuring light at the inclined surface has been proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 2005-109376). Specifically, since the measuring light is refracted at the inclined surface, producing a dark region, the edge of the transparent substrate can be detected. However, it is difficult to make the width of the dark region sufficiently wide by just refracting the measuring light. This causes the problem of being incapable of detecting the dark region reliably.
In addition, a method of causing measuring light to obliquely enter a transparent substrate to produce a dark region by total internal reflection has been proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 2007-165655). However, with this method, if the thickness or refractive index of the transparent substrate changes, the measurement system has to be adjusted. For example, when the refractive index of the transparent substrate has changed, the total internal reflection angle has also changed. Therefore, the incident angle of the measuring light has to be changed. It is not easy for the user to make such an adjustment.
As described above, it has been difficult to detect the edge of a transparent substrate easily and reliably by an optical method.